Ventricular assist devices (VADs) are mechanical pumps that take over the function of a damaged ventricle in a heart failure (HF) or other appropriate patient in order to reestablish normal hemodynamics and end-organ blood flow. In addition, VADs unload the native heart allowing it to rest and, in some cases, the heart can recover function. They can be used as short-term support (days) or as long-term support (weeks or months). VADs can support the right, left or both ventricles. In a left VAD (LVAD) an inflow cannula is connected to the apex of the left ventricle and an outflow cannula is connected to the ascending aorta, whereas in a right VAD (RVAD), the inflow cannula is connected to either right atrium or ventricle and the outflow cannula is connected to the pulmonary artery. The pump can be placed outside the patient's body (extra- or para-corporeal devices) or within the abdomen in a preperitoneal position immediately under the diaphragm or above the diaphragm in the pericardial space (intracorporeal devices).
First generation VADs include pulsatile volume displacement pumps and two valves (outflow and inflow valves). The pumps are driven by either pneumatic or electrical drive systems. Examples of these devices are the commercially available THORATEC PVAD, IVAD, and HEARTMATE XVE, and the no longer commercially available THORATEC HEARTMATE IP1000 and VE, the WORLDHEART NOVACOR and the Arrow International LIONHEART LVD2000.
Second generation VADs include implantable, continuous flow, rotary pumps with axial flow that offer several advantages over the first-generation devices. Some of the advantages are the smaller size that reduces the risk of infections and simpler implantation. There are fewer moving parts, absence of valves to direct blood flow, smaller blood-contacting surfaces and reduced energy requirements that enhance simplicity and durability. These pumps have an internal rotor within the blood flow path that is suspended by contact bearings, which imparts tangential velocity and kinetic energy to the blood. The net action results in generation of a net pressure rise across the pump. An external system driver connected by a percutaneous lead powers the pump. Some of the greatest limitations of this type of device are hemolysis, ventricular suction, thrombus formation and pump stoppage. Examples of these devices are the commercially available THORATEC HEARTMATE II, the JARVIK HEART JARVIK 2000 and the MICROMED HEART ASSIST 5.
Third-generation VADs include centrifugal continuous-flow pumps with an impeller or rotor suspended in the blood flow path using a noncontact bearing design, which uses either magnetic or hydrodynamic levitation. The levitation systems suspend the moving impeller within the blood field without any mechanical contact, thus eliminating frictional wear and reducing heat generation. This feature promises longer durability and higher reliability with low incidence of device failure and need for replacement. Usually, magnetic levitation devices are larger owing to the need for complex position sensing and control system that increases requirements for a large pump size. Examples of these devices are the commercially available TERUMO DURAHEART and the HEARTWARE HVAD, the in development Sun Medical Technology EVAHEART LVAS and the no longer commercially available VENTRACOR VENTRASSIST.
All of the VADs discussed above include a pump external to the patient's vasculature and tubing from the pump to a chamber of the patient's heart, aorta or pulmonary artery. In any case, implantation procedures for such VADs are typically invasive.